VERNON — The Vermont Yankee nuclear plant reported that a failed underground flood seal last week compromised the flooding-prevention design of a nerve center where cables from the plant’s control room are routed to the rest of the plant.

The problems required formal notification to the Nuclear Regulatory Commission.

Vermont Yankee has been shut down since March 9 for refueling and maintenance work, and the problems were in part triggered by maintenance work.

While the so-called switchgear room was not flooded, the 2 feet of water in a manhole in the room was only discovered because the manhole cover was off to allow preparatory work on a new replacement auxiliary transformer, according to Neil Sheehan of the NRC.

Robert Williams, a spokesman for Entergy Nuclear, owner of Vermont Yankee, said the switchgear rooms are where the cables from the control room are routed on their way to the other parts of the plant. The rooms are designed with earthquake protection, as well as fire protection and access controls, he said.

No electrical equipment was flooded, he said.

“The water intrusion was identified and resolved immediately,” Williams said.

Chris Recchia, commissioner of the Department of Public Service, said the issue is important from a policy standpoint because Vermont Yankee had replaced faulty flood seals after Tropical Storm Irene.

“We need to wait for their report and see if it’s a newer problem or an older problem,” said Recchia. “But like the problem last week, I have concern about the aging of the system and what’s required to keep the plant running well.”

If it had been a regular flood, “there could have been serious damage,” he said. “Clearly the system, and the seal, were not functioning well. Whether it was one of the new seals or an old seal, I don’t know.”

Sheehan said the problem started last week when workers, who were cleaning out the Connecticut River water intake structure at the plant, placed watery silt on land adjacent to a manhole. The watery silt created a ponding effect, and the water ran off into the manhole, he said.

That manhole cover seal failed, letting the silty water into the conduit system, and it eventually made its way via a “displaced” conduit pipe seal to a manhole in the switchgear room, said Sheehan.

The water came up 2 feet in the manhole in the switchgear room, but sump pumps were used to remove the water. The manhole itself is 8 feet deep.

“The level of water inside the switchgear room manhole was less than 2 feet at all times and remained well below the level of the switchgear room floor,” Sheehan wrote in an email.

Uldis Vanags, the state nuclear engineer, said Wednesday in a memo to the Vermont State Nuclear Advisory Panel that Entergy was required to submit a licensee event report to the NRC within 60 days of their identification of the degraded flood seal.

“Vermont Yankee has entered this event into their corrective action program and is conducting a cause investigation,” Vanags wrote.

Vanags didn’t respond to a request for further comment.

Williams said the water traveled about 400 feet from the dredging near the intake structure to the switchgear rooms. He said the water did not contact the contaminated areas and was not radioactive.

Sheehan said after the water was discovered, Yankee personnel began inspecting outside manholes and Saturday discovered the mechanical seal that had become “displaced,” which allowed the water to flow from the outside manhole into the switchgear room manholes.

He said the seal was initially reinstalled, but Yankee workers put a new foam seal in place Sunday.

Williams said the failed conduit seal, which did have a mechanical seal threaded into place, was replaced with what he called a better design “to preclude a similar situation in the future.”

Sheehan said the NRC was continuing its post-Fukushima assessment of flooding risks at all nuclear power plants, and that the seals were designed to prevent flooding and remain an area of focus.